Viscosity
Higher the pressure, viscosity increases. Hence the load carrying capacity of the oil increases. This property enables use of thin oils to lubricate heavy machineries.
Viscosity Index
This gives the relation ship between viscosity & temperature of the oil. It is a arbitrary number scale indicating change in oil viscosity with the change in temperature.(VI < 30 low, VI 35~80 medium, VI 80~110 High, VI >110 Very high)
1. A high VI can be expected to undergo very little change in viscosity with temperature extremes.
2. Knowing the VI of an oil is very crucial when selecting a lubricant for an application specially for extremely hot & cold climates.
3. Napthenic oils have very low VI whereas paraffinic oils have high VI.
Pour Point
Pour point is the temperature below which the oil is very resistant to flow/pour.
Select oil to ensure that the pour point is at least 10 deg C lower than the lowest anticipated temperature.
Cloud Point
Certain oils must be maintained at temperatures above the cloud point to prevent clogging of filters. Cloud point is the temperature below which the oil forms wax-like substance.
The cloud point should be as low as possible, thus allowing a wide range of operation.
Flash Point
It determines the volatility & fire resistance of a lubricant. It can also be used to detect potential product contamination (e.g. fuel or chemical contamination)
Flash Point is defined as the temperature at which the oil gives sufficient vapours which give momentary flash upon application of an external source of heat/flame.
The flash point of the LO should be higher than FO.
Total Acidic Number(TAN)
It indicates for oil sample the age of the oil, and can be used to determine when the oil must be changed.
Total Base Number(TBN)
It is the measure of reserve alkalinity in the oil. The knowledge of oil TBN is important when considering its use with fuels of known sulphur content.
Trunk piston oil TBN – 25 ~ 30 mgKOH/ gm
X-head cyl oil TBN 70 ~ 80 mgKOH/ gm
In order to maintain suitable levels of TBN in engine crankcase oil in trunk piston engines, regular freshening of oil is achieved when topping up of oil levels.
Detergency
As the name suggests, this property indicates the cleaning action of the oil. It prevents burnt combustion products from depositing on the piston rings by washing them away with lubricating oil.
Dispersency
It prevents coagulation of particles & deposit elsewhere. By keeping them in suspension & dispersed evenly throughout the oil.
Foaming
This can cause pumps to get air locked, leading to starvation oil to the bearings. Thus, any ideal LO should have resistance to Foaming. It can cause reduction of load bearing capacity of the oil. It can cause cavitation on bearing surface.
Oxidation Stability
Lubricating oil reacts with oxygen to produce weak organic acids. Which is assisted by the presence of active catalyst like Cu, Fe oxide etc. The rate of oxidation increases by two times for 10 deg C rise in temperature.This weak acid leads to the formation of sludge which adhere to the metal surface, forming gum or varnish.
A Special Note
Out of the above listed properties, there are 3 properties which are unique to LO and are not present in Fuel Oil. They are Detergency, Dispersency and TBN. TAN is a complementary parameter to TBN and is used to monitor the LO.
Well, that's all. Adios!